With the recent trend of high-performance semiconductor devices, Cu of low resistivity has been widely used as a wiring material of the semiconductor devices. In manufacturing a Cu multi-layered wiring structure by a damascene process, Cu wiring exposed through an interlayer insulating film may be oxidized because Cu can be easily oxidized. Therefore, a reducing gas such as NH3 or H2 has been used to remove the oxidized Cu by reduction.
However, in case of using NH3 or H2, the temperature of the Cu reduction process has to be high, e.g., 300° C. or higher, and so it is possible that the interlayer insulating film made of a so-called low-k material and formed around the Cu wiring can be most likely damaged. Due to this, low-temperature reduction of Cu by using a processing gas such as vaporized formic acid or acetic acid has been suggested.
However, monomers and dimmers exist together in the vaporized formic acid or acetic acid, which causes unstable reaction. For example, the ratio of the monomers and dimmers formed in the vaporized formic acid or acetic acid is significantly affected by a slight change of conditions, which may result in unstable Cu reduction. (see Japanese Patent No. 3373499).